1. Field of Invention
The present invention relates to a cavity enhanced absorption spectrometer and a method for controlling the same.
2. Description of Related Art
Cavity enhanced absorption spectroscopy (CEAS) is a linear optical absorption spectroscopy. It uses an optical resonator with very low optical loss (e.g. high finesse) to trap circulating optical radiation for many round trips, thus increasing the effective path length traveled by the radiation, typically by a factor of several thousand times the physical length of the resonator. This length enhancement factor also applies to the optical absorption sensitivity, making CEAS an extremely sensitive detection technique. Cavity ring-down spectroscopy (CRDS) is a common type of CEAS in which the optical injection is periodically interrupted, and the exponential ring-down of radiation exiting the CRDS resonator is measured.
CEAS requires injecting optical radiation into the resonator, such as from a laser, typically through one of the mirrors comprising the resonator. The CEAS resonator typically has discrete resonant modes defined by a unique spatial-temporal electromagnetic field profile within the resonator. Because the resonator modes have high finesse (low loss), the radiation spectral acceptance bandwidths of the modes are correspondingly very narrow, much more narrow than the spectral line width of most free-running lasers. Thus only a fraction of the laser power, namely the fraction within a CEAS resonator mode bandwidth (or within the bandwidths of several modes), is injected into the CEAS resonator, and the remaining laser power is wasted.
Also, it is known that the laser optical frequency and CEAS resonator mode resonant frequency quickly drift apart, if not actively controlled together. As a result of this property, many implementations of CEAS sweep the CEAS resonator length (thus sweeping the resonant frequencies of its modes), or the laser frequency, so that the two are momentarily coincident during the sweep, at which time some incident radiation from the laser can enter the CEAS resonator. Sweeping takes time and limits the duty cycle of the CEAS measurement.
To increase the efficiency of injection of the incident radiation into the CEAS resonator, and to avoid the need for sweeping, the frequency of the radiation and the resonance frequency of the CEAS resonator mode are often actively locked to each other. The most common technique of frequency locking is the Pound-Drever-Hall (PDH) technique, which provides a proximity signal (usually called an error signal) used to adjust the frequency of either the radiation source or the CEAS resonator mode to maintain coincidence with the other. Throughout the instant specification and in the appended claims, the terms “Pound-Drever-Hall technique”, “PDH technique” and “technique of Pound-Drever-Hall” shall be understood to mean the technique described in R. W. P. Drever et al, “Laser phase and frequency stabilization using optical resonator,” Appl. Phys. B 31, 97-105 (1983), and E. D. Black, “An introduction to Pound-Drever-Hall laser frequency stabilization,” American Journal of Physics 69, 79-87 (2001), which are hereby incorporated by reference. In accordance with the Pound-Drever-Hall technique, the CEAS resonator mode frequency is usually adjusted by changing the resonator length with, e.g., a piezoelectric transducer (PZT). The laser frequency is usually adjusted by changing its length (in the case of a gas laser or external cavity diode laser (ECDL)) or refractive index (e.g. by current injection in a diode laser or temperature change in a solid state or diode laser). Changing a physical length is relatively slow (bandwidth up to 1 kHz to 10 kHz) since it requires mechanical motion. Changing a refractive index by electronic means is usually fast (bandwidth up to 1 MHz to 1 GHz). Sometimes, the error signal is split into a low-frequency portion which is used to adjust a physical length (either the laser or the CEAS resonator), and a high-frequency portion which is used to adjust the refractive index of the laser.
In addition to CRDS, other forms of CEAS can also benefit from locking. Direct cavity transmission (DCT) spectroscopy and integrated cavity output spectroscopy (ICOS) (see, e.g., A. O'Keefe, J. J. Scherer, and J. B. Paul, “cw Integrated cavity output spectroscopy,” Chemical Physics Letters 307, 343-349 (1999), which is hereby incorporated by reference in its entirety) involve the simple continuous measurement of the radiation intensity transmitted through an optical resonator. These techniques are similar to direct path absorption spectroscopy, the simplest optical spectroscopy, in which a beam of radiation is sent along a path with no resonator, and the transmitted optical power (normalized to incident power) is measured as a function of some parameter such as wavelength. In DCT and ICOS, however, radiation is transmitted only when the radiation is resonant with the optical cavity. As in CRDS, sweeping either the wavelength or the cavity length takes time and limits the duty cycle of the measurement. With locking, the duty cycle is potentially 100%.
To lock effectively, the response bandwidth of the adjustment must be at least as large as the bandwidth of the frequency difference fluctuation between the radiation source and the CEAS resonator mode. This fluctuation is typically dominated by laser frequency noise. In the case of solid-state lasers such as ECDLs, the laser frequency noise bandwidth is usually small compared with the adjustment bandwidth of the laser using injection current. The optical frequency of most distributed feedback (DFB) lasers, however, does not respond quickly to current injection (<1 MHz tuning bandwidth), and their noise bandwidths are often large (>1 MHz). Many other types of lasers lack any fast frequency tuning mechanism at all, such as (most) optically pumped lasers (e.g. Nd:YAG). As a result, there is no direct adjustment of either the CEAS resonator or the laser in the high-frequency range to maintain locking. This reduces the radiation injection efficiency and transient disturbance may cause total loss of lock, including at low frequency. Recovery requires a sweep (or equivalent) to reacquire the lock condition. The reduced injection efficiency reduces the CEAS signal magnitude (thus reducing signal-to-noise ratio), and transient loss of lock introduces gaps in the measurement time sequence. To provide effective locking at high frequency, a laser frequency tuning method other than laser current injection must be employed.